replace "dep" with "go mod"

A simple "go mod init && go mod tidy && go mod vendor" was enough and
mostly seems to have picked the same revisions of the dependencies as
before. Notable exceptions are klog and gogo/protobuf.

vendor/k8s.io/code-generator/cmd/register-gen/args/args.go

@@ -1,39 +0,0 @@
-/*
-Copyright 2018 The Kubernetes Authors.
-
-Licensed under the Apache License, Version 2.0 (the "License");
-you may not use this file except in compliance with the License.
-You may obtain a copy of the License at
-
-    http://www.apache.org/licenses/LICENSE-2.0
-
-Unless required by applicable law or agreed to in writing, software
-distributed under the License is distributed on an "AS IS" BASIS,
-WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-See the License for the specific language governing permissions and
-limitations under the License.
-*/
-
-package args
-
-import (
-	"fmt"
-
-	"k8s.io/gengo/args"
-)
-
-// NewDefaults returns default arguments for the generator.
-func NewDefaults() *args.GeneratorArgs {
-	genericArgs := args.Default().WithoutDefaultFlagParsing()
-	genericArgs.OutputFileBaseName = "zz_generated.register"
-	return genericArgs
-}
-
-// Validate checks the given arguments.
-func Validate(genericArgs *args.GeneratorArgs) error {
-	if len(genericArgs.OutputFileBaseName) == 0 {
-		return fmt.Errorf("output file base name cannot be empty")
-	}
-
-	return nil
-}